A ladder format and mnemonic format are typical expression methods for a sequence program that is executed by means of a programmable controller (hereinafter referred to as PC). Recently, however, a sequence program expressed in a step format, such as an SFC (sequential function chart), has also come to be used.
The sequence program expressed in the step format, unlike the ladder format, is not executed by scanning, but is executed step by step in regular order. When the execution of a final step is completed, the sequence program is finished through, and thereafter, the steps are repeated.
In some cases, the execution of the sequence program in this step format may be expected to be interrupted on account of a machine's failure, checkup of the machine in the middle of programming, etc. The execution can be interrupted either by the method in which the power source of the PC is turned off to restart the processing from the beginning or by the method in which the content of the sequence program is previously set so that the execution can be stopped in a predetermined position.
The method in which the power source of the PC is turned off, however, has a problem such that it requires time-consuming adjustment such as initialization of a machine before resuming a sequence program by reconnecting the power source. Further, in the case where the PC is connected to a network, the power source cannot be turned off without considering its effect.
On the other hand, in the case of the method in which the sequence program is preset for interruption, it is difficult to previously specify a spot at which interruption of the execution is required. Since troubles cannot be foreseen, the execution cannot be interrupted for inspection of faulty spots in the case of this method.